This invention relates to systems for reducing temperatures under the hood of a vehicle. It has particular, but not exclusive, application to such systems for use in a heavy duty truck, more particularly to the tractor portion of a tractor-trailer rig. Heavy duty trucks are typically over-the-road and vocational vehicles considered as weight class 4 to 8. Class 4 is generally considered to include vehicles with a gross vehicle weight of 14,001 lb to 16,000 lb (6350 kg to 7257 kg). The invention is particularly useful with weight class 6 to 8 vehicles, most particularly class 7 to 8 vehicles. Class 6 includes vehicles with a gross vehicle weight of 19,501 lb to 26,000 lb (8846 kg to 11,793 kg). Class 7 vehicles have a gross vehicle weight of 26,001 lb to 33,000 lb (11,794 kg to 14,969 kg).
Vocational heavy duty trucks are adapted for various off-road jobs such as concrete mixer trucks, log-hauling trucks and other trucks which carry heavy loads and are required to operate over rough and uneven ground under off-road conditions. Such trucks are generally also capable of on-road highway travel.
The internal combustion engine of a heavy duty truck produces great quantities of heat. Heat is taken from the engine block by a liquid coolant system, including a radiator spaced in front of the engine block. The heat is dissipated by natural convection, primarily by air flow caused by motion of the vehicle, and by the air which is blown through the radiator and over the engine by an axial fan. In most trucks, the axial fan is run directly from the engine crank shaft, through a clutch. In other vehicles, the fan is electric. In either, the efficiency of the fan is relatively low, frequently on the order of forty percent. This is caused in part by the natural inefficiency of the fan itself. The short distance between the fan and the engine block is also a factor; the engine block forms a natural barrier to air flow and creates a dead space between the fan and the engine block in which positive pressure tends to prevent air flow. A heavy duty truck engine may produce on the order of 1.5 million BTUs (1.6 million kilojoules) an hour at a speed of sixty miles per hour (97 km/hr), producing less than twelve miles per gallon (5 km/l) of fuel usage, and may utilize up to a thirty-two inch (0.8 m) diameter fan nominally pulling 9,500 cubic feet (270 cubic meters) per minute to disperse the heat put out by the radiator. It is estimated that roughly one-third of the output of the engine is utilized to propel the truck, one-third is in the heated exhaust (as heat and unburned fuel), and one-third must be handled by the cooling system.
In addition to the heat carried from the engine block to the radiator by the coolant, the engine block itself radiates substantial heat, estimated to be up to about 20,000 BTUs (22,000 kJ) an hour at highway speeds.
Recently, large diesel trucks have begun to include exhaust gas recirculation (EGR) systems. Trucks equipped with engines over 250 horsepower (186 kW) have consequently begun to experience severe under-hood heat problems. The EGR system recirculates a portion (typically about 15%-30%) of the exhaust gas to the engine air intake. Because the exhaust gas has a temperature of about 1200° F. to about 1500° F. (about 650° C. to about 815° C.), it is cooled to about 600° F. (315° C.) by running it through an EGR heat exchanger before introducing it into the cylinders of the engine. The EGR system adds about thirty percent to the cooling system heat load, about 150,000 BTUs (160,000 kJ) per hour, most of which must be dispersed by the radiator. Because much of this heat is directed back into the engine compartment, under-hood heat loads are dramatically increased. The EGR system itself also radiates heat directly into the engine compartment.
Other auxiliary devices are run by the engine and generate further heat. For example, an air compressor is required for operation of brakes and other components. The air compressor may have a surface temperature on the order of 250° F. (121° C.).
To meet environmental standards and to increase efficiency, a number of other devices have been added to the engine. A turbocharger, powered by the exhaust gasses, generates further heat, estimated to be on the order of 32,000 BTUs (34,000 kJ) per hour. The turbocharger may have an operating surface temperature of about 800° F. to 1200° F. (425° C. to 650° C.).
The additional heat produced under the hood of a modern heavy duty truck has raised temperatures under the hood to unacceptable levels. The additional heat and temperature reduce the lives of components in the engine compartment and reduce engine efficiency. They sometimes melt plastic components and overheat fluids in the engine compartment. The heat also can transfer into the vehicle operator cab and can make both the air temperature in the cab and surface temperatures on the floor and front wall of the cab uncomfortably high. Attempts to enlarge the radiator, by repositioning it, tilting it, or splitting it, have not been sufficient. Enlarging the fan is likewise impractical and would increase the power requirements for running it.
A modern heavy duty truck requires aerodynamic design considerations that sometimes make the hood design smaller. Packing more equipment in the engine compartment further makes airflow more difficult. All of these considerations require a completely new approach to under-hood air management.
Various attempts have been made to solve somewhat similar problems with automobiles and off-highway vehicles. For example, Charles, U.S. Pat. Nos. 4,979,584 and 5,495,909 disclose an automotive engine bay ventilation system using ram air through openings around the radiator of a transverse-engine automobile. Likewise, Corwin et al., U.S. Pat. No. 6,216,778, discloses a cooling system for an off-highway vehicle which is stationary or moves at very low ground speed. These systems are not easily adaptable to the needs of a heavy duty truck.